The basic aim of the proposed research is to characterize stage-specific surface proteins (SSSP) and to examine their role during early development and differentiation of mouse embryo. Syngeneic immunization with embryos of different developmental stages will be used to obtain specific antisera. Surface proteins of preimplantation mouse embryos will be labeled, immunoprecipitated with specific antisera and characterized on one- and two-dimensional gels. Spleen cells from immunized mice will be fused with myeloma cells and hybrids producing monoclonal antibodies against stage-specific surface proteins isolated. Teratocarcinoma stem cell lines will be tested with anti-embryo sera and those with cross-reacting antigens detected. Using monoclonal antibodies and teratocarcinoma stem cells, a sufficient amount of SSSP will be isolated and characterized in detail. Anti-embryo sera will be used to probe the properties of the embryonic cell membrane and to assess the role of SSSP in differentiation and development of embryos and teratocarcinoma stem cells in vitro.